In civilized societies of today, various functions and organisations for maintaining security and safety, referred to as “National Security and Public Safety” (NSPS) organisations, are often dependent on wireless communication for their operation. It is desirable or even a requirement, sometimes, that if the wireless network infrastructure normally used for the communication becomes unavailable, for whatever reason, the mobile terminals used shall still be able to communicate in a so-called direct mode operation (DMO), i.e. communicating directly with each other with no or very limited support and control by the wireless network. This situation may occur during a war-like scenario, or as a result of sabotage or some natural disaster, when the wireless network cannot operate to transmit or receive data to or from the terminals and allocate radio resources for the mobile communication.
FIG. 1 illustrates an example of the normal operation of a mobile wireless access network 100 having various nodes and mechanisms for allocating radio resources to different mobile terminals T1, T2, T3 . . . for either transmission or reception of data over a specific carrier frequency when connected to a base station, not shown, in the network 100. The radio resources are separated in the time domain and arranged as resource elements in basically consecutive time slots TS0, TS1, TS2, . . . of a radio frame structure, e.g. in the manner of an LTE system. Although only one carrier frequency is illustrated here, it can be easily understood that the resource allocation can be differentiated in the frequency domain as well, depending on the frequency bandwidth used.
In this example, the network 100 instruct terminal T1 to transmit or receive data in time slot ISO of a current radio frame. Likewise, terminal T2 is instructed to transmit or receive in timeslot TS3 and terminal T3 is instructed to transmit or receive in timeslot TS5 of the radio frame. The network 100 may employ various scheduling mechanisms for allocating the resource elements, in this case time slots and specific carrier frequencies, to the terminals over time, e.g. by taking into account various factors such as the type of communication, services, priorities and the available capacity in the network, among other things.
Assuming that terminals T1-T3 constitute a group of terminals used by an NSPS organisation and the network 100 would get out of order, e.g. in any of the above situations, the terminals T1-T3 are configured to switch into the DMO in order to maintain important communication within the group of terminals T1-T3. A network type called “TETRA” is known today that can be used for NSPS communication operating both in “infrastructure mode”, i.e. resembling a conventional mobile network controlling the communication, and in DMO. TETRA uses 25 kHz carriers in four simplex channels and the resource allocation in DMO is made according to a quite rigid, pre-defined and thus predictable scheme for determining when and how terminals should transmit and receive data.
However, at least in some of the situations above, the communication may be very sensitive and/or crucial, and it is often desirable that the communication between two terminals cannot easily be tracked by an illicit eavesdropper and/or that an adversary is unable to disturb or “jam” the communication, at least not without great difficulty. Thus, robustness to interference and protection against eavesdropping is often of great importance in NSPS networks. Confidentiality protection can to some extent be achieved by standard encryption techniques. However, a relatively high level of privacy may require that the identity of the transmitting terminal is not disclosed, and/or similarly, that the radio resources used by any specific terminal is unknown.
These issues above have not been solved properly today for mobile terminals when in DMO. The rigid resource allocation scheme of TETRA, as well as other similar network systems known in the art, is predictable and therefore possible to interpret for an illicit party such that eavesdropping and jamming of a particular communication session can be performed without too much difficulty. Conventional techniques such as frequency hopping may be used, but they usually only protect against random, “natural” disruptions in radio propagation and not against adversarial behavior. Moreover, if the network infrastructure is unavailable, there is no entity that can control and co-ordinate the resource usage, which may lead to conflicts, e.g. devices accidentally using the same radio resource, causing interference.